In general, a nitrile group-containing highly saturated copolymer rubber typified by a hydrogenated acrylonitrile-butadiene copolymer rubber is superior in heat resistance, ozone resistance, and other properties to a typical nitrile group-containing copolymer rubber such as an acrylonitrile-butadiene copolymer rubber. For this reason, the nitrile group-containing highly saturated copolymer rubber is used for a wide variety of automotive parts such as fuel oil hoses, O-rings, and in-oil belts.
O-rings incorporated into, for example, automotive engines are used in environments where the O-rings are subjected to fuel oils such as gasoline. Thus, there has been a demand for O-rings having fuel oil resistance to such a degree that they do not swell in contact with the fuel oils. For example, Patent Literature 1 has proposed, as a rubber compound that meets such a demand, a polymer blend containing (i) at least one hydrogenated nitrile rubber, (ii) at least one hydrogenated terpolymer rubber, (iii) at least one salt of a strong base and a weak acid comprising a group I metal, and (iv) at least one olefin/vinyl acetate and/or olefin/acrylate rubber.
Furthermore, O-rings incorporated into, for example, automotive engines used in cold climate areas are required to have cold resistance to such a degree that they have rubber elasticity in low-temperature environments. Providing O-rings having both cold resistance and fuel oil resistance has become a challenge to be addressed. It is generally known that, as a nitrile content of a nitrile-containing highly saturated copolymer rubber is reduced for the purpose of improving cold resistance, fuel oil resistance decreases accordingly. An approach for improving those properties is, for example, controlling a molecular structure of a hydrogenated nitrile rubber, as described in Non-patent Literature 1.